1. Field of the Invention
The present invention relates to data networks and specifically to adjusting link capacity in an intermediate node contained in a data network.
2. Background Information
The Synchronous Optical Network (SONET) standard is an optical network standard that supports multiplexing on links capable of data rates of hundreds of megabits-per-second (Mbps) or more. SONET provides a single set of multiplexing standards for high-speed links at various rates called Synchronous Transport Signal (STS) or Optical Carrier (OC) levels. The basic transmission rate for SONET is STS-1, which operates at a rate of 51.840 Mbps.
SONET is capable of carrying many signals from different services at various capacities through a synchronous, flexible, optical hierarchy. This is accomplished using a multiplexing scheme that multiplexes services into a synchronous payload envelope (SPE) contained within a base STS-1 signal. Services carried by SONET may include voice, high-speed data and video. In a typical arrangement, service data are mapped into e.g., virtual tributaries (VTs) contained within the SPE. VTs exist at a sub STS-1 level within the SPE and are synchronous signals used to transport lower-speed transmissions, such as Digital Signal Level One (DS-1) data, within the SPE.
The SONET optical hierarchy comprises several layers including a section layer, a line layer and a path layer. The Section layer transports STS frames across a physical medium. Its main functions include framing, scrambling, error monitoring and section maintenance. The line layer transports the payload and overhead associated with the path over the physical medium. The line layer provides synchronization and performs multiplexing for the path layer. The path layer transports services between path terminating equipment (PTE) at the ends of a SONET path. The path layer maps signals associated with services into a format required by the line layer.
Each layer in the SONET optical hierarchy is associated with substantial overhead that allows multiplexing and expanded operations, administration, maintenance and provisioning (OAM&P) capabilities. The section layer contains section overhead information (SOH) that is used for communication between, e.g., adjacent elements in a SONET network, such as optical regenerators. The line layer contains line overhead information (LOH) that is used to, e.g., carry STS signals processed by multiplexers contained in a SONET network. The path layer contains path overhead information (POH) that is carried, e.g., on an end-to-end path between a source end (So) of the path contained at a source PTE and a sink end (Sk) of the path contained at a destination PTE. The POH is typically added to signals when they are mapped into VTs and contains information that includes status information associated with the path as well as VT multiframe information.
A SPE may contain one or more VTs wherein each VT is associated with a circuit used to carry e.g., low-speed signal information along a path from a So of the path to a Sk of the path. Moreover, the SPE may use different types of VTs to carry different types of lower-speed signals. For example, a VT-1.5 type VT may be employed to carry a single DS-1 signal and a VT-6 type VT may be employed to carry a single Digital Signal Level Two (DS-2) signal. Further, concatenation may be used to transport payloads that exceed a standard VT type's capacity. For example, several VT-1.5 type VTs may be concatenated to transport payloads that exceed the standard capacity of a single VT-1.5 type VT.
A protocol that may be used to concatenate multiple VTs (circuits) is the Virtual Concatenation (VCAT) protocol. The VCAT protocol is a management plane-oriented protocol that groups, e.g., VTs in a nonconsecutive manner to create virtual concatenation groups (VCGs). VTs belonging to a VCG are called members of the VCG. VCAT uses a control and management plane to establish and manage a path carrying a VCG between a So and a Sk of a path. In a typical arrangement, the control and management plane is used to establish the path for the VCG, identify members of the VCG and assign the path to each of the members.
A VCG is carried over a logical SONET “link” which is a path between a So and a Sk associated with the VCG. Bandwidth associated with VCGs, and hence the capacity associated with the “link,” may be controlled using the Link Capacity Adjustment Scheme (LCAS) protocol. The LCAS protocol enables a VCG's bandwidth to change by e.g., adding or deleting members to and from the VCG in order to increase or decrease the VCG's bandwidth, respectively. For example, the LCAS protocol may be used to to delete a member from a VCG, thereby reducing the amount of bandwidth used by the VCG. Likewise, the LCAS protocol may be used to add a member to a VCG thereby increasing the bandwidth of the VCG. The LCAS protocol is an end-to-end protocol meaning that it is typically implemented at PTE located at the ends of the path, i.e., at the So and Sk of the path.
In many implementations, LCAS support is often provided at the hardware level. Often in these implementations, both the So and Sk of a path must have LCAS-compatible hardware in order to provide “LCAS-like” features and functions. In some data networks, this may require having to replace hardware in the network, which may be costly.
Another problem relates to the availability of hardware that supports LCAS. Assume a vendor provides hardware that supports the LCAS protocol for low speed SONET networks (e.g., a SONET network that operates at a STS-1 rate) but not for high-speed SONET networks (e.g., a SONET network that operates at a STS-192 rate). If an ISP implements high-speed and low-speed SONET networks using the vendor's hardware, the ISP would not be able to provide LCAS support to its customers on the high-speed network. Moreover, even if hardware were available, having to replace existing hardware with new hardware that supports LCAS may create various budgetary issues, as the cost of replacing the hardware may be significant.